Power MOSFETs (Metal-Oxide-Semiconductor Field Effect Transistors) are widely used to facilitate power management and DC/DC conversion in electronic apparatus such as desktop and/or notebook computers, portable devices such as smart phones and power tools, and automotive electronics. When employed in power management applications, the power MOSFETs are usually use as switching devices through which current will through. As such, it is desirable that the ‘On’-resistance of power MOSFETs is as low as possible to minimized power loss as well as heat generation.
Typical power MOSFETS are formed from a plurality of MOSFET cells which are connected in parallel. For example, in conventional planar VDMOS (Vertical Double-Diffused MOS) transistors, a basis cell consists of two vertical transistors sharing a common drift region and controlled by a common gate portion which extends across the common drift region. In general, a large common drift region will mean a smaller JFET resistance. If the drift region is not wide enough, the on-state resistance of the transistors will be high. On the other hand, the overall gate capacitance in conventional VDMOS is dependent on the width of the drift region.
The channel of a MOSFET is typically formed by lateral diffusion of dopants underneath the polysilcon gate at a high temperature and by a long thermal cycle. The high temperature and long duration of a thermal cycle process will generate a deep body junction and a non-uniform lateral doping profile underneath the gate. As a result, severe dopant out-diffusion from substrate may also occur.
Furthermore, a deep body junction and severe dopant out-diffusion from substrate require a thicker epitaxial (‘epi’) layer to sustain the same blocking voltage, and a non-uniform lateral doping profile will require a longer channel length to sustain the same punch-through breakdown voltage. On the other hand, MOSFETs having a long channel length and thick epitaxial layer thickness will result in a high on-resistance and a high power loss in high current application.
It would be beneficial if MOSFETs having improved designs are provided.